1. Field of the Invention
The present invention relates to gas turbines. More specifically, the present invention relates to the air bled from the compressor section of a gas turbine for use in cooling the turbine section and provides a method for cooling such air by evaporating water in the flow of such air.
2. Description of the Prior Art
A gas turbine is comprised of three main components: a compressor section in which air is compressed, a combustion section in which the compressed air is heated by burning fuel and a turbine section in which the hot compressed gas from the combustion section is expanded. To achieve maximum power output of the gas turbine, it is desirable to heat the gas flowing through the combustion section to as high a temperature as feasible. Consequently, the components in the turbine section exposed to the hot gas must be adequately cooled so that the temperatures of the materials which form these components are maintained within allowable limits.
This cooling is achieved by flowing relatively cool air over or within the turbine components. Since such cooling air must be pressurized to be effective, it is common practice to bleed a portion of the air discharged from the compressor section and divert it to the turbine components for cooling purposes. Although the cooling air eventually mixes with the hot gas expanding in the turbine, since it bypasses the combustion process, not all of the work expended in compressing the cooling air is recovered in the expansion process. Consequently, to maximize the power output and efficiency of the gas turbine, it is desirable to minimize the quantity of cooling air used.
Unfortunately, as a result of the temperature rise which accompanies the rise in pressure in the compressor, the air bled from the compressor is relatively hot, 600.degree. F. to 800.degree. F. depending on the compression ratio. Hence, it is well known in the art that the quantity of air bled from the compressor for cooling purposes can be reduced by cooling the air prior to directing it to the turbine components, thereby increasing its capacity to absorb heat.
One method commonly used to cool the cooling air utilizes and air-to-air cooler, whereby the air bled from the compressor flows through finned tubes over which ambient air is forced by motor driven fans, thereby transferring heat from the compressed air to the atmosphere. Although this method achieves adequate cooling, it suffers from two significant drawbacks. First, since the system requires an air cooler, interconnecting piping, structural support members, fans, motors and associated electrical controls, it adds significantly to the cost of the gas turbine. The second drawback concerns performance. The heat extracted from the compressed air in the cooler is lost to the atmosphere, thereby detracting from the thermodynamic efficiency of the gas turbine. In addition, the power required to drive the fans must be subtracted from that produced by the turbine, thereby reducing the net power output of the gas turbine.
A second method, used with gas turbines operating in a combined gas and steam turbine cycle system, employs an air-to-water cooler. According to this method, the air bled from the compressor flows over tubes in which boiler feedwater flows, thereby transferring heat from the compressed air to the feedwater. Although this method recovers the heat removed from the compressed air and returns it to the cycle, and hence does not suffer from the performance disadvantage of the method discussed above, it involves the considerable expense of an air-to-water cooler, subjects the turbine to damage from flooding in the event of a tube failure and requires a water circulating system for periods when the boiler is out of service.
It is therefore desirable to provide a system and method for cooling the air bled from the compressor for cooling purposes which is inexpensive, simple to operate, reliable and does not detract from the performance of the gas turbine.